1. Field of the Invention
The invention is directed to a piston ring for a piston of a piston-cylinder unit such as a vibration damper.
2. Description of the Related Art
Flat pistons for vibration damper piston-cylinder units in which a piston ring surrounding a piston has a substantially U-shaped cross section are known from the prior art. The piston ring forms a jacket which seals a gap between the piston and an inner wall of a cylinder of the vibration damper. Supporting surfaces of the piston ring serve to secure the piston ring axially on the top and bottom of the piston, respectively. The piston ring is often made from a rigid plastic so that radial pretensioning may be achieved between the piston ring and the inner wall. A slight gap between the piston and piston ring in the area of the circumferential surface of the piston typically exists because the dimensional tolerances of the cylinder and piston must be balanced.
In a flat piston disclosed, for example, in DE 197 07 633, massive deformation processes may cause inaccuracies in the shape of the piston ring so that the supporting surfaces of the piston ring no longer make full surface contact with the top and bottom of the piston. This may lead to the occurrence of small leakage gaps between the top and bottom of the piston and has a negative impact particularly in vibration dampers for so called sport car designs which require precise damping characteristics in that the flow connections which are subject to tolerances generate variations in damping force.
One possible solution consists in readjusting the piston in a final work step to eliminate the dimensional inaccuracy. Although this yields measurably successful results, it is still not sufficient. Further, the tolerances at the piston ring are limited, but at the expense of a disproportionate increase in manufacturing costs with only a small reduction in the permissible manufacturing tolerances.
The reference cited above describes the possibility of sealing the gap between the jacket of the piston ring and the inner wall of the cylinder by a seal integrated in a seal carrier.